The present invention relates to a gas cleaning process and apparatus for removing suspensions of solid and/or liquid particles (i.e., aerosols) entrained in a gas stream which utilizes a wetted, electrostatically charged filter media. The present invention is particularly suited for cleaning gaseous effluents emitted from various industrial installations such as incinerators, calciners, utility boilers, sulfonation operations and wood products manufacture facilities, among many others.
Electrostatic precipitation is a widely used technique for separating solid and liquid aerosols from gas streams. Electrostatic precipitators are characterized by at least one ionizing electrode (e.g., a wire, sharp-edged rod or other conducting member having a small radius of curvature) maintained at high electric potential and spaced from one or more ground or precipitating electrodes of relatively large surface area. Particles entrained in the gas to be treated are charged as the gas is forced to pass through the limited current discharge (i.e., corona) between the ionizing and precipitating electrodes. The electric field drives the charged particles to the collecting region of the apparatus where they are discharged and precipitated on the surface of the precipitating electrode.
The collecting surfaces of an electrostatic precipitator must be freed of precipitated material from time to time in order to maintain the desired collection efficiency. As the aerosol load in the gas to be treated increases, more frequent cleaning of the collecting surfaces of the precipitator is necessary. If the particles being collected are essentially dry, removal of precipitated material can be achieved by rapping or shaking the precipitating electrodes. In applications where the particles being collected are wet and/or tacky, a wet electrostatic precipitator design may be employed. In wet electrostatic precipitators, the collecting surface of the precipitating electrodes is a liquid film. The liquid film, usually aqueous, may be provided by precipitation of droplets entrained in the gas being treated and/or by irrigating the precipitating electrodes with a liquid spray. During operation, the film of liquid continuously drains from the precipitating electrodes of a wet electrostatic precipitator, thereby removing collected solids which would otherwise tend to accumulate. Although wet electrostatic precipitators are capable of achieving high collection efficiencies, even with respect to smaller (e.g., submicron) diameter particles, the associated capital and operating costs are often prohibitive.
Cloth bags, commonly referred to as baghouse filters, are used to remove solid particles entrained in dry gas streams. As dust-laden gas flows into the filter bag, entrained solids collect on the bag and clean gas passes through. Periodically, the collected material is dislodged from the bag by mechanically shaking the bag or by flexing the bag with a reverse pulse of compressed air. Baghouses are simple and relatively inexpensive to operate and can achieve high collection efficiencies. Unfortunately, baghouses are not suited for cleaning gas streams having a high liquids content and/or containing tacky solids since it is difficult to remove collected material from the bags. Moreover, in some designs, it may be necessary to interrupt the gas cleaning operation while collected material is being removed from the filter bags.
Venturi and other types of scrubbers can be used to remove liquid particles and tacky solids from gas streams. However, to achieve high collection efficiencies, especially with respect to smaller particles, high pressure drops must be used leading to increased operating cost.
Therefore, there remains a need for a system for continuous, efficient cleaning of gaseous, industrial effluents capable of achieving a high degree of removal of solid and liquid aerosols entrained in the gas.